RF frequency modulation that is wideband (WB), linear and of fine precision, while consuming low power, is an area of intensive research. It is used in many applications, including cellular mobile polar transmitters. The principal advantage of polar transmitters over the conventional IQ transmitters is power efficiency since the information signal at the power amplifier input has a constant envelope and therefore can be efficiently processed by a highly compressed non-linear amplifier. The amplitude modulation, in this case, can be generated by modulating the amplifier's bias or a number of active MOS switches.
Polar transmitters, however, suffer from a unique set of challenges. The frequency modulated (FM) clock is generated by the phase-lock loop (PLL), whose frequency is controlled by analog voltage on a varactor or a digitally controlled capacitor in the LC-tank. Irrespective of the implementation, the frequency gain as a function of the input control (continuous or discrete) has to be precisely known or, better yet, constant over the entire FM range for a distortion-free transmission. The existing polar transmitters work reasonably well for narrow-band (NB) modulation schemes, such as GMSK and 8PSK (EDGE). This is not the case, however, for WB modulated signals, which have large frequency excursions near zero-crossings in the IQ constellation. If the FM signal is discrete-time then the FM excursions will be as high as ±fs/2, where fs is the sampling rate of the FM signal at the modulator input. Such wide bandwidth signals impose another challenge in discrete FM modulators or digital-to-frequency converters (DFC), which has to do with quantization and linear FM range.
There is thus a need for a low power digitally controlled oscillator (DCO) capable of wideband frequency modulation having fine precision suitable for use in a digital polar transmitter. Designing a DCO to synthesize wideband FM signals such as WCDMA and LTE poses two main challenges: (1) linearity of the DCO transfer function, and (2) resolution of the DCO over the entire FM range. Meeting these constraints in an RF oscillator with switched capacitor array implementation is difficult because of the inherent nonlinear resonant frequency versus tank capacitance curve over a wide frequency range. In addition, such implementations, depending on the desired FM tuning range, requires a large number of unit cells in the capacitor bank which would increase routing complexity and would also impact the Q-factor of the LC tank.